1. Field of the Invention
The invention relates to driver circuitry for driving recording elements for non-impact printing and more particularly relates to driver circuitry for driving light-emitting diodes (LEDs) on a printhead with improved compensation for nonuniformities in light outputs arising from temperature differences on the printhead.
2. Description of the Prior Art
In the prior art, as exemplified by U.S. Pat. No. 4,885,597, the contents of which are incorporated herein by this reference, an LED printhead is described for recording by modifying an electrostatic charge on a photoconductive web. LED printheads, as known in the art, include a single row of LEDs formed in chip arrays. Each array may contain say 192 LEDs. As may be seen from FIG. 1, the arrays 31 are mounted end to end on the printhead 20 so that a single row of several thousand LEDs 30 is provided. Driver chips 40 may be mounted on one or both sides of this row of LEDs and these chips incorporate the circuitry for handling the data signals to determine which LEDs are to be turned on or illuminated during a pixel (picture element) recording period and for providing the driving currents to the LEDs turned on. In one typical application, two driver chips incorporate the circuitry for driving the 192 LEDs on a chip array. One of these driver chips will be used to drive the 96 odd-numbered LEDs and the other will be used to drive the 96 even-numbered LEDs. Signal carrying lines 33, 34, 35, 36 and 37 are provided to carry signals from a logic control unit and power supply to control operation of the printhead.
As noted in this patent, temperature gradients appear along the length of the printheads. The LEDs are temperature-sensitive and their respective light outputs for the same amount of driving currents are adversely affected. Thus, illumination uniformity suffers and concomitant image deterioration results. To offset this, the referenced patent teaches the use of modifying the driver currents respectively in accordance with temperature measured close to the driver chips to compensate for the change in light output due to the LEDs rise in temperature. In accordance with the circuit described in this referenced patent, a digitally addressable current regulation circuit is provided on each driver chip. This circuit drives the LEDs through a current mirror with the currents through the LEDs being slaved to that in the master circuit. The current in the master circuit is regulated with a digital word stored on the driver chip. This digital word is changed periodically with changes of temperature, thereby causing change in the level of current in the master circuit, and concomitantly, in the level of current to the LEDs. This approach works well when temperature changes are relatively slow so that infrequent changes to the digital word may be made. However, where exacting image reproduction is desired, the circuit described in this patent presents some problems. In order to change the digital word more frequently, communication with say a logic controller must be had requiring lines and attachment connections. Additional lines and connections to a printhead use up precious "real estate" on the printhead. To minimize the use of this real estate, it has been proposed in U.S. application Ser. No. 07/543,929, filed Jun. 26 1990 in the names of Potucek et al to employ data lines for passing image data signals to the printhead for also passing digital current regulation data. While this approach alone saves "real estate", the passing of current regulation data may be required only during interframes for recording or other nonproduction intervals. While an embodiment disclosed in the Potucek et al application describes a circuit for accomplishing line by line changes to both image data and current regulation data, such circuit may require a higher speed clocking capability than desired. The Potucek et al application also overcomes the problem of temperature-related changes in LED uniformity by providing a circuit including a resistor having a positive temperature coefficient in combination with a bipolar transistor. Incorporating such a resistor on a so-called V.sub.T (thermal voltage) reference source provides a current that increases with temperature. This current is then used as a reference for generating the master current in the driver circuit for driving the LEDs. Thus, as temperature of the driver chip increases, and concomitantly its adjacent LED array, the current to the LEDs increase to offset reduction in efficiency due to temperature.
A problem with the above approach is providing a reference current that increases linearly with temperature. Such an increase is desirable since it provides a reasonably satisfactory approximation of the requirements of the LEDs for maintaining uniformity of light output. A further problem is that it would be desirable to provide such a reference current source that has a temperature coefficient that is programmable to provide further flexibility in allowing adjustments to be made to best approximate the behavior of the LEDs. For example, LEDs produced from different batches may exhibit uniformity fall-off at different rates. A circuit that can be programmed to offset this fall-off with good approximation would be highly desirable.